There are three typical systems for ink-jet printing. In one system ink droplets are ejected through a small nozzle normally from several microns to several hundreds microns in diameter, and each droplet is imparted a predetermined static charge by electrical signals, then deflected electrostatically between high-voltage deflection plates to form a character or figure on a recording medium according to a predetermined dot matrix. In a second system ink droplets are drawn through a small nozzle of the above specified size under the electrostatic control of high-voltage plates, accelerated, and then electrostatically deflected between deflection plates to form a letter or figure on a recording medium according to a predetermined dot matrix. In the third system, ink droplets are ejected in response to electrical signals to form a letter, figure or a picture on a recording medium according to a predetermined dot matrix.
Therefore, the basic concept of ink-jet printing is to form a letter or figure on a recording medium by ink droplets ejected through a small nozzle, charged electrically and controlled electrostatically, or by ink droplets discharged from the nozzle in response to electrical signals.
Aqueous ink for use in ink-jet printing comprises water (as a vehicle), a hydrophilic organic solvent, a coloring dye and other optional additives. With such ink, a letter, figure or picture can be printed on various objects such as plastics, metals, paper or cloth. Having these applications, aqueous ink for ink-jet printing must meet the following requirements.
(1) Requirements for providing uniform ink droplets through a small nozzle:
(a) the ink has a viscosity of from 1 to 10 cPs at 20.degree. C.; PA1 (b) the ink has a surface tension of more than 20 dynes/cm; and PA1 (c) the ink comprises uniformly sized particles. PA1 (a) the ink droplets applied onto the recording medium dry quickly enough to form a letter, figure or picture that does not lose shape; PA1 (b) the ink is sufficiently waterproof to provide a letter, figure or picture on the recording medium that does not dissolve in water; and PA1 (c) the ink has good tack to the recording medium. PA1 (a) the ink contains dyes that provide colors identical with those required in the final color images; PA1 (b) for color production by the subtractive process, the ink preferably contains yellow, magenta and cyan inks which absorb only light in the desired spectral ranges and desirably do not absorb light in other ranges; PA1 (c) the ink provides a color image which is satisfactorily sharp and does not spread on the recording medium; PA1 (d) the ink contains dyes that provide high chroma and color density, and its hue and light-fastness do not change during ink storage or after it is applied onto the recording medium to form an image; PA1 (e) the ink contains dyes that have a water solubility high enough to provide an ink composition of high chroma and color density; PA1 (f) the ink contains dyes that do not make the ink extremely viscous; and PA1 (g) the ink contains dyes that have low crystallizability and which do not crystallize or solidify during storage to form a deposit that clogs the nozzle.
(2) Requirements for providing a good image on a recording medium:
(3) Requirements for providing a good color image on a recording medium:
The water-soluble dyes conventionally used in ink for ink-jet printing are direct and acid dyes of the type described in Japanese Patent Application (OPI) No. 89534/74 (the symbol "OPI" as used herein means an unexamined published Japanese patent application), Japanese Patent Publication Nos. 16243/79, 16244/79, 16245/79, Japanese Patent Application (OPI) Nos. 96105/77, 146307/77, 77706/78, Japanese Patent Publication No. 21765/79 and Japanese Patent Application (OPI) No. 89811/79. However, ink using these dyes is not completely satisfactory because it sometimes clogs the nozzle during continuous operation or when the operation is restarted after a short or long suspension, or it provides an image that spreads on the recording medium or which is not highly light-fast. As a further disadvantage, the combination of yellow, magenta and cyan ink compositions incorporating these dyes does not always provide a color image having faithful color reproduction.